1. Field of the Invention
The invention relates to the art of foamed polymers and more particularly pertains to isocyanurate foams prepared by the polymerization of methylene-bridged polyphenylpolyisocyanate mixtures.
2. Description of the Prior Art
The preparation of isocyanurate foams by the polymerization of polymethylene polyphenylpolyisocyanates is well-known. Generally, these polymers are prepared by mixing together a polymethylene polyphenylpolyisocyanate mixture, an inert blowing agent, a trimerization or isocyanurate group formation catalyst, and a modifying or stabilizing agent. The resulting foam is rigid, has relatively high thermal stability and can be used for thermal insulation, a building material, and the like. Examples of some prior art isocyanurate foams and methods of preparation are described in U.S. Pat. Nos. 3,745,133; 3,644,232; 3,676,380; 3,168,483; and 3,516,950, to name a few.
However, the heretofore known polyisocyanurate foams based on polymethylene polyphenylpolyisocyanate mixtures have suffered from one or more disadvantages, such as being susceptible to flame spreadability, high friability, irregular cell structure, low adhesiveness and the like. Due to one or more of these disadvantages, the prior art polyisocyanurate foams have been less than satisfactory and leave much to be desired.
We have now found that it is possible to prepare modified polyisocyanurate foams that do not exhibit any of the aforesaid disadvantages. We have unexpectedly discovered that, by the utilization of mixtures of methylene-bridged polyphenylpolyisocyanates containing specific amounts of the 4,4'-, 2,4'-, and 2,2'-isomers of methylene diphenylisocyanate and having an average functionality of about 2.1-2.4, modified isocyanurate foams can be produced that have excellent thermal stability, uniform cell structure, excellent adhesion and low friability. The overall properties and details of preparation of these foams are unexpectedly superior to those foams disclosed in the prior art.